Recently, attention has been paid to a technique of composing a semiconductor integrated circuit or a TFT using a semiconductor thin film on an insulating substrate such as a glass or quartz substrate. The TFT is widely applied to electronic devices such as IC and an electro-optical device, and has been rapidly developed especially as a switching element of an image display device.
There are various applications of such an image display device such as a digital video camera and a liquid crystal television, and the image display device is expected to be applied to mobile electronics such as a cellular phone, a portable game machine, a portable television, or a portable terminal especially for the future. As characteristics required by users for these mobile electronics, points of being light and being durable in order not to break when dropped, for example, are given.
However, the substrate used for the previous image display device is a substrate comprising an inorganic material such as a glass substrate or a quartz substrate, as described above, and there are defects of breaking and being heavy, which are unique to the inorganic material. In order to overcome the defects, the formation of a TFT on a substrate with plasticity, typified by a flexible plastic film, has been tried.
Compared to the substrate such as the glass or quartz substrate, however, the substrate such as the plastic film has low heat resistance, and therefore, a processing temperature in manufacturing a TFT is limited. In the result, it was difficult to manufacture a TFT directly on the plastic film, which has favorable characteristics compared to the TFT formed on the glass or quartz substrate. Consequently, a high-performance image display device or light-emitting device that uses a plastic film has not been realized.
Recently, a peeling method for peeling a layer to be peeled existing over a substrate with a separating layer interposed therebetween, has already been proposed. For example, Japanese Patent Laid-Open No.10-125929 and Japanese Patent Laid-Open No. 10-125931 describe a technique of separating a substrate by providing a separating layer of amorphous silicon (or polycrystalline silicon) and irradiating a laser beam through the substrate to release hydrogen contained in the amorphous silicon to form pores. In addition, Japanese Patent Laid-Open No.10-125930 gives a description of sticking a layer to be peeled (in the publication, called a layer to be transferred) on a plastic layer with the use of this technique to complete a liquid crystal display device.
However, since amorphous silicon or polycrystalline silicon is used as the separating layer in the aforementioned method, a problem is considered that an irradiated laser beam is transmitted through the separating layer depending on a film thickness thereof and a wavelength of the applied laser beam to damage the layer to be peeled. Further, in the aforementioned method, in the case of forming an element on a separating layer, hydrogen contained in the separating layer is diffused and decreased when a heat treatment at a high temperature is carried out in the process of manufacturing the element. As a result, there is the possibility of insufficient peeling even if the separating layer is irradiated with a laser beam. Therefore, there is a problem that the process after forming the separating layer is limited in order to retain of the amount of hydrogen contained in the separating layer. Additionally, it is difficult to peel a layer to be peeled with a large surface according to the aforementioned method. Although the aforementioned publication gives a description of forming a light-shielding layer or a reflection layer in order to prevent the layer to be peeled from being damaged, in that case, it becomes difficult to fabricate a transmission type liquid crystal display device.